The invention relates generally to powered motive systems incorporating bearings such as director systems and servo control systems and more particularly to low friction and low stiction motive systems in conjunction with electric motors as power sources and incorporating bearings which support both the motors and the load.
In certain precision applications it is desirable to have a motive system output which can operate smoothly throughout its operating range in order to provide a high degree of operational predictability and accuracy. Various types of systems have been utilized which are designed to reduce friction in the bearings used in the system and thereby provide smooth, consistently accurate system operation. In this regard, it is important to note that the friction and "stiction", i.e., resistance to start up motion, of the bearing for the load support and the friction and stiction of the bearing for the driving power system are additive so that these undesired bearing effects cumulatively contribute to the overall system imperfections.
Some precision applications employ servomechanisms to accurately control the position of an output shaft. An example of such a servomechanism is disclosed in Jamieson U.S. Pat. No. 3,034,028. The Jamieson device is a position control servomechanism which has an input member which provides an electrical signal utilized to control an output member. The Jamieson invention employs frequency modulation techniques to correlate the speed, shaft position or other measurable characteristic of the input and output members. Although the Jamieson device can achieve precise control of the output member, it does not address the undesirable effects friction and stiction may have on the performance of the device. In this regard, it is well known that friction will tend to disturb the smooth operation of a servo controlled motor system, and this is particularly true of stiction, the often severe resistance to start up motion. A system with stiction often suffers from undesired motion, such as jitter, that often negates effective servo control.
In an effort to minimize friction in various types of motors (electric, turbine etc.), many designs have been devised which purportedly have reduced friction in such motors. One such design utilizes a bearing incorporating air under pressure. Although this air bearing design is very effective in reducing friction, its disadvantage is that it must be initially set into motion by other means before its low friction advantages can be fully realized. Consequently, stiction remains an important shortcoming of this design. Although magnetic suspension designs have essentially eliminated stiction, such designs are very expensive and are probably limited to fairly small systems.
In an attempt to overcome the limitations of conventional motor systems presented by their inherent stiction and friction characteristics, some prior art motor systems have incorporated preloading in order to negate the effects of these forces of friction and stiction. However, such designs tend to adversely affect the smooth operation of the motor system. In addition, compensating for the stiction and friction characteristics of a motor system makes the system inordinately complex making it more error prone as well as more likely to malfunction and break down.
Many prior art systems incorporate two motors which are simultaneously controlled. An example of such a system is disclosed in Hender U.S. Pat. No. 3,771,032. The Hender invention is basically an electric motor control system in which two motors which are interconnected electrically and mechanically are electrically controlled so that the armatures and field assemblies can rotate at different speeds in order to afford infinitely variable speed control of members driven jointly by them. Although utilization of two motors effectively allows variable speed control of the driven members, the necessity of utilizing two motors would, in many applications, increase the stiction of the system.
In many modern commercial applications, it is desirable to control the speed of the driving power system. Some prior art motor systems employ either a means of introducing resistance in the wound rotor circuit or a special control circuit to vary the speed of the driving motor. An example of such a system is disclosed in Drummond U.S. Pat. No. 2,787,747. The Drummond system incorporates a pair of similar motors in which the rotors are interconnected and in which the stators may be mechanically interconnected for opposed synchronous rotation. However, a major disadvantage of the Drummond system is that it does not address the increased friction presented by the use of two motors and does not address the increased stiction also presented by the use of two motors which may introduce undesired motor output control imprecision when the system is started.
Some prior art motor systems utilize two interconnected motors in order to provide greater (or specific) control of the motor torque output. An example of such a motor is disclosed in Carlson U.S. Pat. No. 4,373,147. The Carlson motor incorporates two motors mounted within a single housing. One member of each of the motors rotates relative to the other motor member. The first motor rotor and the second motor stator in the Carlson invention are rigidly secured together. The Carlson device is specifically adapted to measure the consistency of the material in which the output is immersed. The Carlson motor determines the consistency of the material by measurement of the reaction torque experienced by the first stator which feeds a control signal to the second stator to compensate for the reactional torque experienced by the first stator. However, the Carlson device is an accurate measuring unit only if the rotor is not moving, and very inaccurate if the rotor is moving due to the effects of back EMF, friction etc. In addition, the Carlson motor generally has the same number and type of component parts as two separate motors and does not present any savings in weight or complexity over two separate motors . Furthermore, the Carlson patent does not address the problems of friction and stiction in a servo controlled application.
A motive system incorporating bearings is thus needed that can substantially reduce the effects of bearing friction and stiction and result in a controlled system that is free from undesired jitter for enhanced output precision and consistency. A motive system incorporating bearings is also needed that can achieve reduced bearing friction and stiction while having fewer component parts for improved simplicity, reliability and efficiency.